The present invention relates generally to fastener-driving tools used for driving fasteners into workpieces, and specifically to combustion-powered fastener-driving tools, also referred to as combustion tools or combustion nailers.
Combustion nailers are known in the art for driving fasteners into workpieces, and examples are described in commonly assigned patents to Nikolich U.S. Pat. Re. No. 32,452, and U.S. Pat. Nos. 4,522,162; 4,483,473; 4,483,474; 4,403,722; 5,197,646; 5,263,439 and 5,713,313, all of which are incorporated by reference herein. Similar combustion-powered nail and staple driving tools are available commercially from ITW-Paslode of Vernon Hills, Ill. under the IMPULSE® and PASLODE® brands.
Such nailers incorporate a housing enclosing a small internal combustion engine or power source. The engine is powered by a canister of pressurized fuel gas, also called a fuel cell. A battery-powered electronic power distribution unit produces a spark for ignition, and a fan located in a combustion chamber provides for both an efficient combustion within the chamber, while facilitating processes ancillary to the combustion operation of the device. Such ancillary processes include: mixing the fuel and air within the chamber, turbulence to increase the combustion process, scavenging combustion by-products with fresh air, and cooling the engine. The engine includes a reciprocating piston with an elongated, rigid driver blade disposed within a cylinder body.
A valve sleeve is axially reciprocable about the cylinder and, through a linkage, moves to close the combustion chamber when a work contact element at the end of the linkage is pressed against a workpiece. This pressing action also triggers a fuel-metering valve to introduce a specified volume of fuel into the closed combustion chamber.
Upon the pulling of a trigger switch, which causes the spark to ignite a charge of gas in the combustion chamber of the engine, the combined piston and driver blade is forced downward to impact a positioned fastener and drive it into the workpiece. The piston then returns to its original or pre-firing position, through differential gas pressures created by cooling of residual combustion gases within the cylinder. Fasteners are fed magazine-style into the nosepiece, where they are held in a properly positioned orientation for receiving the impact of the driver blade.
The cooling fan motor is housed in the cylinder head of the tool, and the fan blade is attached to a fan motor shaft, which passes through a hole in the cylinder head. It is preferred that the clearance between an inside diameter of the hole and the motor shaft outer diameter is kept to a minimum to prevent the unwanted leakage of combustion pressures during the drive stroke to increase tool power. At the same time, the shaft needs to freely rotate for proper fan operation, slide axially relative to the cylinder head to absorb impact forces generated in combustion, and avoid frequent contact with the edges defining the hole. The latter problem can result in hole widening or unwanted noise generated during operation.
Since the piston return cycle is relatively long, 5 to 10 times the duration of the power stroke, and relatively low pressures are used for piston return, less than −5 psi (gage) compared to greater than 85 psi (gage) during combustion, it is a goal of tool designers to conserve pressure escapement through the clearance area between the motor shaft and the hole. If pressure loss is substantial enough, at best, piston return times will increase, and at worst, the piston may not return. If piston return times are longer than the time it takes for the operator to open the combustion chamber to atmospheric pressures, piston return will cease and no nail will subsequently be driven.
Another design consideration of such nailers is that it is preferable for managing motor shock and displacement to allow venting between the motor and the cylinder head during the drive stroke. Venting prevents combustion pressures from acting on the motor surfaces that urge the motor outboard of the tool.
Therefore, there is a need for an improved combustion nailer which addresses the above-identified design parameters, including maintaining venting around the motor during the drive stroke, and preventing or minimizing leak paths during the piston return cycle.